Out of Africa migration may be a no-brainer

It was not big brains alone that set the first humans migrating out of Africa to Eurasia, according to a surprisingly small fossil skull unearthed at a rich site in Dmanisi, Georgia. It had been suggested that early human migration began when an intellectual threshold - recorded as increased skull size - was crossed.

The skull is the third to be found at the 1.75 million year old site and is considerably smaller than the earlier skulls. It suggests that early Dmanisi humans did not differ so dramatically from their African contemporaries, meaning a single trigger for human migration is unlikely.

The new find also combines features found in a number of different early human species. This further erodes the textbook view that human evolution was a linear progression from Australopithecines through Homo habilis, Homo ergaster and Homo erectus and provides fuel for continued debate between anthropologists. For example, the team reporting the new skull believe Homo ergaster and Homo erectus are the same species.

The debate is certain to continue, comments Harvard anthropologist Daniel Lieberman: "The great value of finds like these is the questions they raise, not the answers they provide."

Sex or age differences?

The new skull has a brain volume of 600 cubic centimeters, putting it in the range of Homo habilis, yet the Dmanisi skulls also have erectus-like features says team member Reid Ferring of the University of Northern Texas in Denton, . But the researchers have still "provisionally" assigned all three skulls to Homo erectus.

The diversity at Dmanisi shows "hominid variation at that time was much greater than we ever suspected," Ferring told New Scientist. Yet we can recognise that diversity only because the site has yielded more early human skulls from the same time than any other site - it is impossible to gauge diversity at a site that yields only one skull.

The reason for the differences between the fossils is unclear. Team leader David Lordkipanidze, at the Georgian State Museum in Tblisi told New Scientist that they may be male-female differences.

But Milford Wolpoff of the University of Michigan, who has seen the skulls, says the diversity "comes with age, because two [smaller skulls] aren't adult, as well as with sex, because one mandible is a big old male." Wolpoff says all three skulls are female.

Journal reference: Science (vol 297, p 85)

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